In the Universal Mobile Telephony System (UMTS), defined by the standards published by the 3rd Generation Partnership Project (3GPP), a mobile device, or user equipment UE, can establish links with several cells in the cellular radio network.
The data received by the user equipment from each of the cells is transmitted in frames on respective downlink channels. The frames in the respective downlink channels are not completely synchronized with each other. Moreover, because of possible movement of the user equipment relative to the base stations in the different cells, the timing of each downlink frame, relative to the other received downlink frames, can change.
The user equipment transmits data in frames on the uplink to the base stations in the cells with which the user equipment has links. Thus, there is defined for the user equipment a transmission frame timing, which is common to all of the uplink channels. This transmission frame timing is defined with reference to the downlink frame timing of one of the active links, termed the reference cell.
Furthermore, there is defined a receiver window relative to the transmission frame timing. The user equipment is able to receive information from any base station system whose downlink transmission falls within the defined receiver window. The receiver window is defined in the 3GPP technical specification TS 25.133 as T+/−148 chip before the transmit timing. The base station systems with which the user equipment has active radio links are defined as the active set. In use, for example as the user equipment drifts further from or nearer to particular base station systems, some radio links may be lost, and new radio links may be set up. The reference cell against which the transmission frame timing is defined can be removed from the active set, at which time a new reference cell can be defined. If the active set comprises more than one cell which have downlink frame timing inside the receiving window of the user equipment, the user equipment is in soft handover or softer handover.
A user equipment can be in soft handover for long periods of time in CDMA based mobile communication systems. If the timing is simply updated towards a reference cell, when a plurality of cells exist, the user equipment may push one or more cells outside the receiver window. Also, when a plurality of cells exists, selecting a new reference cell to update timing towards could cause unnecessary large adjustments of the transmit timing. Large adjustments are undesirable as they could cause performance degradation both for the user equipment and the network.
Drifts in the timing may also occur when the user equipment is stationary. A slow drift takes place even when the user equipment is stationary since there is a small frequency difference between the clocking of the base station and the user equipment. If the timing is simply frozen while the drift continues, the user equipment will eventually lose sync with the cells in the active set when the paths from all cells have drifted out of the receiver window. This problem can be visible in a stationary long duration connection in soft handover if the reference timing is frozen.
It would be advantageous to have a system and method for updating user equipment timing that overcomes such disadvantages and limitations.